Pipeline pigs are used to remove coatings or scale from the inside of pipelines. This coating can vary in thickness and hardness. For example, deposits of coke can form soft coatings several millimeters thick, while hard scale such as iron sulphide may form coatings less than 1 mm thick. Pipeline pigs are forced through the pipelines under hydraulic pressure and the coating is removed by the scraping action of the pigs. To improve the scraping action, such pigs may include hard appendages that scrape the coating. These appendages are subject to wear, and can be expensive and inconvenient to replace.
In my co-pending application I disclose a pipeline pig having a generally cylindrical body made of elastic material by a moulding process, and a plurality of appendages disposed about and extending radially outward from the body. The appendages are secured to the body by being threaded into threaded sleeves that are inserted in the body during the moulding process, with plates extending laterally outward from the sleeves into the body to resist removal of the appendages from the body and to prevent the appendages from being driven into the body. By being threaded into the sleeves, the appendages may be readily detached from the pig body.
I have now designed an improvement to the pipeline pig disclosed in that application, in which the central portion of the body has a portion with reduced radius such that at least a first set of appendages extends radially outward less than a second set of appendages. Thus when the pig is in a pipeline and being moved along the pipeline by hydraulic pressure, the first set of appendages will not be worn as much as the second set of appendages and their lifetime will be extended. When a part of the pipeline is encountered which is coated with an unwanted deposit, the pig will find it harder to move through that part of the pipeline and hydraulic pressure will build up behind the pig, until the pig is forced through the coated part of the pipeline. The hydraulic pressure on the pig will cause the central portion of the pig to expand, bringing the first set of appendages against the pipeline and thus in position to scrape the coating from the interior of the pipeline.
To allow replacement of worn appendages, the appendages are also readily detachable. Preferably the appendages are secured in the body of the pig by being threaded in sleeves, with the sleeves having laterally extending flanges at their bases. The appendages may include a hardened tip for use in the case of hard scale. Washers may be used to extend the appendages outward as they wear.
For applications where little by-pass of the hydraulic fluid past the pig is desired, the ends of the pig are preferably flared and hollow so that hydraulic pressure on the pig will cause the flared ends to expand into sealing contact with the interior of the pipeline. The greater the resistance that the pig encounters, the greater will be the expansion of the flared end sections and the central portion of the pig.